1. Field of the Invention
The invention relates to improvements in the production of materials based on modified starch present in an intimate admixture with selected thermoplastic polymer compounds and to molded articles manufactured from said materials.
2. Discussion of Related Art
Numerous recent proposals deal with the attempt of opening up an extended range of possible fields of application to high molecular weight polymer compounds of natural origin--and especially to starch, among these. All of this work has been based on the recognition that native starch, together with limited amounts of water and/or further auxiliary materials, by means of a thermomechanical digestion can be converted into a thermoplastic material that can be processed in a conventional manner, for example by injection molding. The thermomechanical digestion by employing elevated temperatures and pressures is possible, more specifically, in conventional extruders provided upstream of the shaping step. From the comprehensive literature, here particular reference may be made to the publication by R. F. T. Stepto et al., Injection Moulding of Natural Hydrophilic Polymers in the Presence of Water, Chimia 41 (1987), No. 3, 76-81, and the literature quoted therein.
Extensive work deals with the improvement of the product properties in the thermoplasticized starch by the concomitant use of selected organic auxiliary liquids in the starch digestion. Here, as an example, the PCT patent application No. WO 90/05 161 may be mentioned. Herein it is proposed to add loading materials to the native starch, which loading materials decrease the melting temperature of the starch and are additionally characterized by a definite solubility parameter. As the loading materials there have been especially mentioned lower polyfunctional alcohols such as ethyleneglycol, propyleneglycol, glycerol, 1,3-butanediol, diglyceride, appropriate ethers and further compounds.
Materials based on starch which has been thermoplasticized in that manner are distinguished by a very much restricted utility. The reason is the extremely marked hydrophilicity of the material. Accordingly, the replacement nowadays desired in wide areas of polymer compounds of synthetic origin by a biologically degradable polymer material of natural origin is possible only to a very limited extent or is not even possible at all.
More recent proposals relate to the attempt of combining thermoplasticized starch with synthetically produced water-resistant polymer compounds in such a manner that the hydrophilic starch-based polymer compound exhibits an increased stability against hydrophilic solvents, while neverthelesss substantial proportions of the materials or molded article, respectively, are comprised of the starch. From the pertinent literature, the PCT patent application No. WO 90/01 043 may be mentioned. Described therein is coating hydrophilic polymers, such as starch, with selected aliphatic polyesters which in turn are degradable, especially by bacteria or fungi. In particular there is described coating molded articles made of starch with polyesters derived from lower hydroxycarboxylic acids, such as glycolic acid, lactic acid, hydroxybutyric acids and hydroxyvaleric acid. Since such layers when combined do poorly adhere to each other, an improvement of the adhesion is proposed by some preceding treatment of the starch surface and/or by the addition of a solvent or swelling agent for the starch to the coating solution.
Another approach--towards the same goal--is described by EP-A2-327 505. Herein, polymer mixed materials have been described which are recovered from a melt of water-containing destructured starch and at least one water-insoluble synthetic thermoplastic polymer compound. The procedure in detail is as follows: First, a thermoplasticized starch is produced--and worked up to form granules--from starch in an untreated or pretreated form with the addition of auxiliary materials such as hydrogenated fatty acid triglycerides, lecithin and especially water by a treatment in an extruder at elevated temperatures--of, for example, 175.degree. C.--and the associated intrinsic elevated pressures. The water content in the granule is adjusted to the water content of natural starch (about 17% by weight). This granular starch is then mixed in the dry state with synthetic polymer compouds in pre-determined mixing ratios. Examples of such thermoplastic water-insoluble polymers of synthetic origin are polyethylene, polystyrene, polyacetals, ethylene/acrylic acid copolymers and ethylene/vinyl acetate copolymers. The respective polymer mixtures are then processed in a conventional manner on an injection molding machine at material temperatures of 165.degree. C. to form molded articles.
Further proposals in the area concerned here of thermoplastically processable starch-based polymers are found in EP-A1-0 400 531 and EP-A1-0 400 532 and in the PCT patent application No. WO 90/10 671. The last-mentioned printed publication, more particularly, contains extensive information on the procedures of mixing and thermoplastically digesting the polymer compounds employed in the presence of water in suitable extruders and on the procedure of at least partially stripping the water from the mixed materials, expediently in the process step carried out in the extruder.
The subject matter of the older German patent application P 40 38 732.1 comprises materials and/or molded articles based on thermomechanically digested starch in admixture with synthetic thermoplastic polymer compounds. These polymer-modified materials are produced by mixing native starch with aqueous polymer dispersions of the synthetic thermoplastic polymer compounds and, if desired, further lower molecular Weight plasticizers, subjecting the multi-material mixture to a starch digestion at elevated temperatures and pressures with simultaneous intensive mixing and/or kneading to form a thermoplastically processable starch, and optionally shaping the homogenated polymer mixture. The amount of water introduced via the aqueous dispersion of the synthetic polymer compound is an integral constituent of the process which is utilized and becomes effective in the digestion process of digesting the starch. Suitable, at least largely water-insoluble thermoplastic synthetic polymer compounds, according to this older proposal by Applicants, are, for example, emulsion (co)polymers such as polyvinyl ester, poly(meth) acrylates and/or corresponding copolymers. Polyesters, polyamides and/or polyurethane resins have further been mentioned, among which those thermoplastic polymer compounds may be preferred which contain polar groups or molecule moieties and optionally connect to molecule moieties having a marked oleophilic character.
It is the subject matter of a further older German patent application by Applicants according to P 41 21 111 to modify materials and/or molded articles based on an accordingly thermomechanically digested starch in such a manner that there are present, in admixture with the thermoplasticized starch, suitable polyesters and/or polyamides as synthetic thermoplastic polymer compounds which have been at least partially composed of key substances based on re-growable raw materials.
The further development now presented as described hereinbelow resumes Applicants' proposals set forth in the two above-mentioned older German patent applications according to P 40 38 732 and U.S. application Ser. No. 08/070,409, now U.S. Pat. No. 5,439,953 and modifies the teaching thereof in the way described hereinbelow. For the sake of completeness of the disclosure of the invention, the contents of the disclosures of U.S. application Ser. No. 08/070,409, now U.S. Pat. No. 5,439,953 is explicitly incorporated by reference in the present invention.
The teaching of the present invention comprises the following concept: The thermomechanically digested starch is to be present in admixture with selected thermoplastically processable polymer compounds, which exhibit oxidative reactivity due to selected functional groups. The production of the polymer mixture from thermoplasticized starch and the incorporated synthetic polymer compound is effected in the manner described in Applicants older application, that is with the exclusion of oxygen and/or air, respectively, to such a degree that an oxidative reaction of the incorporated oxygen-reactive polymer component does not or not substantially take place. Obtained is a thermoplastically processable starch-based polymer mixture which may either be directly shaped in a per se known manner or is storable for a virtually unlimited time under the exclusion of oxygen--or air--access. The molded body made from this polymer mixture and brought into the desired three-dimensional shape is then, more particularly, exposed to the action of air. The oxidatively reactive groups of the incorporated polymer component are, thus, activated and lead to cross-linking or at least partial cross-linking of the synthetic polymer material having been homogeneously admixed with the thermoplasticized starch. The most important result of this reaction consists of that the resistance to water of the starch-based polymer material is positively influenced. Thereby the utility of the corresponding polymer mixtures is substantially enhanced. Further details will be found hereinbelow.